Far Side Astronomy: Orbiting Telescope Would Use Moon as Shield

Researchers have submitted the Dark Ages Radio Explorer to NASA for consideration to probe the first stars and galaxies that began to light the universe.

Credit: University of Colorado

Out of darkness, there came the blazing light of the very first stars and galaxies in the universe — and the best place to look back at them might be the far side of the moon.

The proposed Dark Ages Radio Explorer (DARE) telescope would orbit the moon, using the lunar bulk as a shield from the noisy radiation of Earth as the instrument probed those early days of the universe. The ambitious project is currently under review for NASA funding.

Even behind the protective shield of the moon, DARE would face a challenging task: It would be attempting to collect light from sources 13.6 billion light-years away while contending with the glare of the Milky Way galaxy. The telescope would be operating in an environment with rapid temperature fluctuations and uneven gravity threatening to yank the craft out of orbit. The DARE team must demonstrate to NASA that it can overcome all those challenges to be selected. [How the DARE Dark Ages Radio Explorer Would Work (Infographic)]

"This is a challenging experiment because we're having to look through a foreground — namely, the [Milky Way] galaxy — that is at least four orders of magnitude brighter than the signal that we're trying to detect," said Jack Burns, DARE's principal investigator and the director of the Lunar University Network for Astrophysics Research at University of Colorado Boulder.

"We really did a deep dive this last year, in particular into some of the thornier engineering issues and software issues," he told Space.com.

During the universe's dark ages, space was filled with a haze of neutral hydrogen gas. Turbulence in the gas clouds made them collapse down into the first stars, or maybe black holes, and the radiation they generated blasted away the gas and gave it a charge (this period is known as the epoch of reionization).

The farther you look, the earlier eras you can see in the universe. But those dark ages foil a general probing because the gas hides the very first objects to form. So, instead of looking at those burgeoning lights, DARE aims to look at the neutral hydrogen around them, to work out what those first objects must have been like and the radiation they gave out, Burns said.

"Scientifically, we've got a problem that is really a juicy one," Burns said.

"This is really the only technique that we know of that allows us to probe the true first generation of stars," he added — out to as early as 80 million years after the Big Bang.

Deep dive

Light from that era has what astronomers call a "redshift" of 35. The universe is expanding, and so the light given off by that neutral hydrogen — which starts out as a 21-centimeter-long wavelength (about 8 inches) — will be more and more stretched out the farther it travels. That effect, redshift, lets researchers estimate how long ago light was emitted. But at the distances DARE is investigating, the manageable 21 cm becomes very faint and incredibly stretched — all the way into the realm of FM radio. That, plus the distortion the Earth's ionosphere wreaks on incoming radio waves, makes it very difficult to find the signal among the noise from Earth, Burns said.

Hence the search for a quiet spot on the moon's far side. To make the most of that advantage, Burns' group has been fine-tuning its experiment to provide better shielding from sunlight and a more easily calibrated antenna. The researchers have also been integrating new ways to analyze the data they collect and finding a rare, stable "frozen orbit" for the spacecraft.

To search for those faint signals during each of its 2-hour orbits, DARE would take observations when it passes behind the moon, taking its main observations when both the sun and moon are out of its field of view. Then, it would send its information back during periods when it was on Earth's side of the moon. (The instrument could also analyze the sun during more of its off time, Burns said.)

DARE's measurements need to be incredibly precise, and minuscule changes to the antenna's size due to temperature can affect that precision, Burns said. To reduce the impact as the spacecraft passes into and out of the sunlight, the researchers have added a sunshade to the telescope to protect the antenna and have switched to materials similar to carbon fiber that expand and contract very little with temperature. The researchers also designed a process to double-check the antenna's measurements once DARE is in space: They'll send a very precise set of frequencies from a radio telescope on Earth for DARE's radiometer to measure. If there's been any change to the antenna properties or the measurements are skewed at all, the researchers could then compensate for it.

The researchers are also adding a polarimeter, which is a device that measures the way light is polarized. While the background radiation is slightly polarized, the signal's light is unpolarized, which makes it easier to pick out, Burns said. Polarized light oscillates slightly more often in a certain direction, while unpolarized light vibrates in all directions equally.

And even after those ultraprecise measurements and markers, like the particular wavelength and unpolarized nature of the light, coaxing the signal from the Dark Ages' neutral hydrogen among the much brighter radiation from everywhere else will be a challenge, Burns said.

"We're just subtracting the sky, and what's left will be the signal," he said. "Having said that, it's nowhere near that simple."

Burns' group has spent the past five years developing software that uses a process called Bayesian inference to let researchers interpret data in this low signal to noise regime. The researchers employ a similar process to what LIGO uses to detect the tiny wobbles of gravitational waves. The DARE group's algorithm simultaneously fits the foreground, signal and instrument conditions to the data and any prior information and measurements to tease out the most probable cause of the signal. [The Universe: Big Bang to Now in 10 Easy Steps]

Fundamental

DARE is an engineering challenge, but a space-based telescope shielded from Earth is the only way to look that far back into the dark ages, Burns said. (Ground-based radio telescopes, like HERA, can't see quite as far back.) A launch vehicle would put DARE into high-Earth orbit, and then the spacecraft would use its own propulsion system to get situated around the moon, where the telescope would begin to answer questions about the transition to the stars and galaxies seen today.

"How did the first galaxies form in the early universe?" Burns said. "That's pretty fundamental, because those first stars and galaxies led to a second and a third generation of stars like our sun, and ultimately to the Earth and life. What we're studying is the very first part of that chain that eventually led to us."

Buns said the team will also be able to settle a long-standing debate about the early universe: whether the first denizens to emerge from the dark ages were indeed gigantic stars, or whether black holes could have come first. If extra-dense patches had formed smaller black holes — from tens to a hundred solar masses — before the first stars, the signatures on the neutral hydrogen would look different, Burns said. (And the universe would have really gotten into the dark-ages spirit.) Either way, researchers would learn more about the first black holes and the large, bright stars that first sparked light.

And there's one more thing the researchers discovered along the way to DARE: a new orbit around the moon that won't require frequent readjustment. That was a pleasant surprise, Burns said.

When large meteors and asteroids strike the moon, they become embedded in the bottom of a crater and subtly throw off the moon's gravity, because they're denser than the surrounding lunar material. It's happened enough that the moon's gravitational pull isn't constant in a smooth sphere around the moon, so satellites in a standard orbit would eventually destabilize without being constantly adjusted.

Burns' group used detailed maps of the moon's gravity, collected during NASA's GRAIL moon probe mission, to find a better orbit, which is elliptical: 50 kilometers (30 miles) above the surface on the far side and 120 km (75 miles) above on the near side. In that orbit, DARE — or other future spacecraft, like tiny cubesats observing the moon with limited propulsion power — could persist in orbit for much longer, Burns said.

"Shoot-out"

Burns' team submitted DARE to NASA last September as part of the agency's Medium-Class Explorers (MIDEX) program, and by midsummer, the agency should select two or three of the submitted projects to fund concept studies — a $ 2 million budget for nine months to further the concepts needed to make DARE a reality. If it's selected, the DARE team will write close to 1,000 pages in a detailed report for NASA's consideration documenting the project.

"Then there'll be a competition, kind of a 'shoot-out,' between the final missions, and various review panels will look at it [on site], and then just one will be selected for flight," Burns said. "It's a tough competition. It's all or nothing, is what it really comes down to. There is no second place."

But Burns said he is confident that DARE is situated to provide a unique, and essential, view of the universe's origins.

"We've got a dynamite scientific problem. We've got the ideal location — namely the moon and the far side, where it's proven to be radio-quiet. We've got technological solutions now that I think are first-rate for all the major hurdles, and finally, the software," he said.

"In my mind, this is the last frontier in observational cosmology," he added. "It's the fundamental thing we do not yet know or have probed, that epoch of the first stars and galaxies."

Sarah started writing for Space.com in June of 2015. Her work has been featured by Scientific American, IEEE Spectrum, Quanta Magazine, Wired, The Scientist, Science Friday and WGBH's Inside NOVA. Sarah has an MA from NYU's Science, Health and Environmental Reporting Program and an AB in mathematics from Brown University. When not writing, reading or thinking about space, Sarah enjoys musical theatre and mathematical papercraft. You can follow her on Twitter @SarahExplains.